1. Field of the Invention
Methods and arrangements for recovering a rare-earth phosphor from the sediment in the drain channels of a color-television tube screen-coating room.
2. Description of the Prior Art
The viewing screen of a color-picture tube is comprised of a plurality of regularly arranged groups of three round or line-like phosphor dots which, depending on the type of phosphor used, emit green, blue or red light, respectively, when bombarded by electrons. Each color is excited by one out of three electron guns contained in the picture tube.
In the most widely used method of making such color picture screens, the phosphors are applied to the screen photochemically. An aqueous suspension containing the phosphor to be applied and, as a photosensitive material, polyvinyl alcohol with ammonium bichromate, for example, is applied to the screen. A thin coating settles down, excess solution being removed by decanting and collected. From this solution, the phosphor is recovered by means of a centrifuge. The applied coating is dried and then exposed to light through a shadow mask having circular holes or an "aperture grille". In the exposed places, the polyvinyl alcohol becomes insoluble in water and binds the embedded phosphor at the surface of the screen. In the unexposed places, the coating is removed by rinsing, and the rinsing water with suspension dissolved therein is collected. This is done for each of the three colors. The phosphors are usually applied in the order green-blue-red, mostly using copper-activated zinc-cadmium sulfide for green, silver-activated zinc sulfide for blue, and an europium- or samarium-activated rare-earth oxysulfide, such as europium-activated yttrium oxysulfide, for red. In former years, use was also made of zinc selenides and zinc-cadmium selenides and of rare-earth oxides and vanadates.
Only a small portion of a phosphor is mounted on the screen by the exposure to light, while the greater portion is subsequently rinsed out again, so considerable amounts of phosphor are left as residues. As the ratio of the prices per kilogramme of the phosphors green, blue and red is approximately 1:0.5:10, there is a special interest in recovering at least the red phosphor. Since the red phosphor is deposited last, the rinsed-out red phosphor from the unexposed portion of the screen unavoidably includes portions of the previously deposited phosphors. To recover this red phosphor, a number of methods are known, such as those disclosed in U.S. Pat. No. 3,474,040 and German Published Patent Application (DT-OS) No. 2,126,893.
Both in the centrifuge and with any of the above-mentioned methods of recovery, a certain percentage is lost by being carried off in the sewers. To this must be added the phosphor which is washed out from scrap faceplates and scrap envelopes. At the current production rates of color-picture tubes, therefore, so much sediment including red phosphor accumulates in the drain channels of a screen-coating room that, in view of the high price per kilogramme of the red phosphor, recovery from this sediment would be of great advantage. To our knowledge, this has never been done before.